Mainly that it appears to be a word of fairly recent coinage...possibly something that wouldn't be used in everyday speech. But then one could argue that the same has been true of the English "introvert" until fairly recently in our history...while the term has been around for a while, it really wasn't until Myers-Briggs that the word came to be part of the vocabulary of the average guy on the street.

If you truly want the noun "introvert," I think it's the best option. If you really mean someone who is shy or a loner, however (not a correct definition of "introvert," but it does sometimes get misused that way, so confusion is understandable), we need to know that.

Redwolf

It doesn't sound natural in Irish. Perhaps with a lot of usage it would become accepted and understood in speech. Since the word 'indíritheoir' is in the dictionary Foclóir Gaeilge Béarla (FGB), translators may give this as a translation for 'introvert' in text. The dictionary Foclóir Gaeilge Béarla is over 30 years old, so we have a 'language committee' on http://www.focal.ie and at http://www.gaeilge.ie which decides on terminology, which translators can correctly give.

It seems 'indíritheoir' is around for a long time.... so who am I to argue with it, but I would say 'Seán is an introvert' as:
Tá luí (beag) lena chomhluadar féin ag Seán.
an introvert:
duine a bhfuil luí (beag) lena chomhluadar féin aige

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Bí cinnte de go nglacfaidh triúr le gach aistriúchán a thabharfar.
Be sure to get three in agreement with a translation given.

Thanks, you've all been very helpful! I appreciate it.